Digital transmission networks, such as those based on Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) standards, are used extensively for transporting broadband communications signals. Network elements, such as multiplexers, digital cross-connect systems, and the like, are used in these transmission networks to support a number of different applications, including some that involve multiple switching or routing functions. It is to be understood that the terms "switching", "routing", "selecting" and "connecting" are used herein interchangeably to refer to the handling of signals within a transmission path. One example of an application with multiple switching functions is "path-in-line" protection switching, also referred to as "virtual rings" or "ring-on-ring", which involves line switching over bi-directional line switched rings (BLSR) and path switching over unidirectional path -switched rings (UPSR). To support these types of applications, some network elements include a routing structure, such as a switch fabric, to provide the necessary connections for switching signals through the transmission network for a given network application. A centralized switch fabric, i.e., a single, homogeneous switch fabric, is one example of a type of switch fabric that is commonly used.
Generally, the problems with prior art control arrangements for centralized switch fabrics relate to the complex coupling of control functions. Using digital transmission networks as an example, fault detection control for incoming input signals is typically based on signal status derived from signal monitoring in the port interface of a network element. In order to make appropriate selection decisions within the switch fabric based on signal status, the fault detection control functions associated with each of the monitored input signals must be coupled to the switch fabric control function. Some of the more notable problems with this configuration are inefficient and time consuming exchanges between the various control functions and switching delays as a result of the extensive coordination required between control functions. These problems are compounded when a switch fabric supports hundreds or thousands of input and output lines in a network element. In particular, the switch fabric control function must be coupled to the fault detection controls for each individual input signal, including signals from any previous selection points. Therefore, status of signals must be traced back to their respective input and resolved before another selection is made. This complex resolution process results in long analysis delays at each selection point, thereby decreasing the performance of the network element.
An alarm indication signal is used in some types of networks, e.g., SONET, for alerting downstream equipment that an upstream defect has been detected. However, an alarm indication signal is a separate maintenance signal and is not used to retain signal status, e.g., quality information, about a particular input signal. As such, an alarm indication signal is not used to propagate signal status through the network for individual input signals, and, as a result, signal status for each input signal must still be "rediscovered" at each subsequent switching point using some type of signal monitoring function. In addition, an alarm indication signal is not generated for all of the failure indicators used within SONET and is not structured to accommodate non-SONET signal status information. Given these limitations, a control arrangement based on an alarm indication signal-type scheme does not provide an effective means for resolving the status of signals transported through switching points.